<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <!-- 
      order: 决定item在容器中的排列顺序
             值为(正整数、负整数、0)
             排列顺序是从小到大
  -->
  <style>
    .container{
      height: 150px;
      display: flex;
      border: 1px solid;
    }
    .container div{
      width: 150px;
      background-color: #ccc;
      border: 1px solid;
    }
    .container :nth-child(1){
      order: 0;
    }
    .container :nth-child(2){
      order: 2;
    }
    .container :nth-child(3){
      order: -1;
    }
  </style>
  <div class="container">
    <div>1</div>
    <div>2</div>
    <div>3</div>
  </div>
</body>
</html>